The present invention is generally directed to horizontal penetrations extending between the inner and outer walls of a cryostat, particularly one employing liquid helium as a coolant material. More particularly, the present invention is directed to an insert for this penetration which employs a large plurality of foam spheres for insulation and blowout protection. Even more particularly, the present invention is directed to a cryostat insert for horizontal penetrations in which electrically conductive leads extend from the penetration in normal operation (that is, non-retractable leads).
In the generation of medical diagnostic images in nuclear magnetic resonance imaging, it is necessary to provide a temporally stable and spatially homogenous magnetic field. The use of superconductive electrical materials maintained at a temperature below their critical transition temperatures provides an advantageous means to produce such a field. Accordingly, for such NMR imaging devices, a cryostat is employed. The cryostat contains an innermost chamber in which liquid helium, for example, is employed to cool the superconductive materials. The cryostat itself typically comprises a toroidal structure with other nested toroidal structures inside the exterior vessel to provide vacuum conditions, intermediate liquid nitrogen cooling and thermal shielding. Since it is necessary to provide electrical energy to the main coil magnet, to various correction coils and to various gradient coils employed in NMR imaging, it is necessary that there be at least one penetration through the vessel walls. Typical prior art penetrations have been vertical. However, from a manufacturing and utilization viewpoint, the construction of vertical penetrations has produced undesirable problems of alignment, assembly and size. However, horizontal cryostat penetrations have not been employed for reasons of thermal efficiency. In particular, it is seen that for a coolant such as liquid helium, that there is a large dependency of density upon temperature. Accordingly, liquid helium vapor found within a vertical penetration is naturally disposed in a layered configuration as a result of density variations from the bottom to the top of the penetration. This layering provides a natural form of thermal insulation along the length of a vertical penetration. In particular, at any position along the axis of such penetration, the temperature profile is substantially constant. However, this would not be the case for a horizontal cryostat penetration since any layering that would result would not be in a direction of the long axis of the cryostat penetration. Accordingly, the temperature gradient along the penetration would tend to set up free convection currents in the vapor within the penetration. This would result in a much more rapid loss of coolant than is desired. Since the cost of helium is relatively high, it is seen that the loss of coolant is undesirable.
Moreover, as a result of an as not yet fully understood phenomenon, it is possible for superconductive windings within a cryostat to undergo a sudden transition from the superconducting state to the normal resistive state. In this circumstance, the electrical energy contained within the coil is rapidly dissipated as resistive (I.sup.2 R) heating of the windings. This can result in a rapid increase in internal helium vapor pressure and accordingly, cryostat penetrations should usually be provided with pressure relief means. Furthermore, vacuum conditions are maintained between the innermost and outermost cryostat vessels. If for some reason, a loss of vacuum occurs in this volume, it is also possible to develop a rapid increase in the coolant vapor pressure. For this reason also, pressure relief means are desirable for cryostat penetrations.
As indicated above, electrical connection must be provided through the cryostat wall to accommodate the electrical apparatus contained therein at the desired lower temperature. In some cryostat penetration designs, the electrical connections to the internal coils are made through an electrical lead assembly which is disposed entirely within an inner cryostat vessel. In such a configuration, there is a tendency for frost buildup upon the contacts and these contacts often must be heated to a temperature of about 300.degree. K. prior to making an electrical connection to them. It is, of course, undesirable that interior cryostat objects must be heated. It should also be understood that because of the superconducting nature of at least some of coils disposed within the innermost cryostat vessel, a persistent current mode of operation is intended. In such a mode, once desired currents are established, the electrical power supply to the electrical elements within the innermost vessel can be disconnected. This is an advantageous mode of operation since it is highly energy efficient. However, it is seen that this mode of operation exhibits the disadvantage that the electrical leads may have to be heated to provide the desired electrical contact, particularly during start-up excitation of the magnet. However, many of these problems are avoided by providing a non-retractable electrical lead assembly disposed within the penetration. However, the utilization of such a non-retractable assembly introduces insulation, convection current and pressure relief problems which are not present in the retractable lead cryostat design.
Accordingly, it is seen that because of the large density changes between cold and warm helium, vapor free convection secondary flows are easily set up in horizontal cryostat penetrations. These flows considerably degrade the thermal efficiency of the horizontal penetration. If the penetration is densely packed with foam or equipped with a vapor cooled, thermally efficient blowout plug, pressure relief could be obstructed by frost buildup in the vapor cooled channel. It is therefore seen that horizontal cryostat penetrations for NMR magnet cryostats require thermally efficient inserts that suppress free convection vapor flows. These inserts must also provide sufficient exhaust area to relieve internal vessel pressure in case of magnet quench or vacuum loss. Additionally, these inserts must also accommodate non-retractable electrical leads.